[BUG] Grid objects for Custom filters on the columns are getting lost or overwritten

Describe the bug UI Grid custom filters sample implementation is present at the page http://ui-grid.info/docs/#!/tutorial/Tutorial:%20306%20Custom%20Filters

If you open this sample implementation and also open console logs. When you apply filter initially it works with no error in logs. If you now again click to apply another filter, as soon window opens you see error in logs however filter gets applied successfully on main grid which is fine. But this error leaves us a question that it could not found the earlier object hence error was thrown each time filter pop up window opens.

To Reproduce Steps to reproduce the behavior:

1. Go to ' url mentioned in the description and go to demo'
2. Click on 'filter in the Age column apply some filter then again reopen filter pop up '
3. Scroll down to ' console log'
4. See error

Expected behavior This error does not happens initially it only happens successively. I would not expect this error as object for grid to filter on column is somewhere already there in the memory

Screenshots If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• Windows 10
• chrome browser
• Version [e.g. 22]